The present invention relates to impingement cooling of a gas turbine nozzle band side wall adjacent an undercut region of a nozzle segment and particularly relates to impingement cooling of the nozzle band side wall in a design where the weld joint between the nozzle segment cover and the nozzle side wall is remote from the nozzle wall exposed to the hot gas path.
In current gas turbine designs, nozzle segments are typically arranged in an annular array about the rotary axis of the turbine. The array of segments forms outer and inner annular bands and a plurality of vanes extend between the bands. The bands and vanes define in pan the hot gas path through the gas turbine. Each nozzle segment comprises an outer band portion and an inner band portion and one or more nozzle vanes extend between the outer and inner band portions. In current gas turbine designs, a cooling medium, for example, steam, is supplied to each of the nozzle segments. To accommodate the steam cooling, each band portion includes a nozzle wall in part defining the hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therewith and an impingement plate disposed in the chamber. The impingement plate defines with the cover a first cavity on one side thereof for receiving cooling steam from a cooling steam inlet. The impingement plate also defines, along an opposite side thereof and with the nozzle wall, a second cavity. The impingement plate has a plurality of apertures for flowing the cooling steam from the first cavity into the second cavity for impingement cooling the nozzle wall. The cooling steam then flows radially inwardly through cavities in the vane(s), certain of which include inserts with apertures for impingement cooling the side walls of the vane. The cooling steam then enters a chamber in the inner band portion and reverses its flow direction for flow radially outwardly through an impingement plate for impingement cooling the nozzle wall of the inner band. The spent cooling medium flows back through a cavity in the vane to an exhaust port of the nozzle segment.
The cover provided in each of the outer and inner band portions is preferably welded to the corresponding nozzle side wall. In prior designs, the weld joint between the cover and the nozzle wall was disposed at a radial location between the nozzle wall and the spline seal between side walls of adjacent nozzle segments. In that location, the weld joint was exposed to the high temperature gases in the hot gas flow path and the side wall was very difficult to cool. Thus, weld joint fatigue life was significantly reduced due to its proximity to the hot gas path. Moreover, the location of the weld was not optimum for manufacturing repeatability and was very sensitive to manufacturing tolerances. The weld joint was characterized by variable wall thicknesses which increased the stress at the joint, decreased the low cycle fatigue and limited the life of the parts. The wall thickness at the weld after machining was also a variable which could not be tolerated in the manufacturing process.
In accordance with a preferred embodiment of the present invention, a cooling system is provided in a nozzle segment design in which the weld joint between the cover and nozzle side wall is on the side of the spline seal remote from the nozzle wall exposed to the hot gas path. That is, the weld joint between the cover and the nozzle side wall of the outer band is located radially outwardly of the spline seal between adjacent outer bands while the weld joint between the cover and the nozzle side wall of the inner band is located radially inwardly of the spline seal between adjacent inner bands. This reduces the temperature of the weld joints during turbine operation, reduces the stresses across the joints, both thermal and mechanical, eliminates any requirement for machining after welding and results in joints of constant thickness and higher fatigue life. The location also leads to improved machinability and tolerance to weld defects.
To provide that weld location, undercut regions adjacent the side walls of each nozzle segment bands are formed. Particularly, each undercut region includes a side wall or edge of the nozzle segment and an inturned flange extending inwardly from and generally parallel to the nozzle wall (in the hot gas path) and spaced from the nozzle wall. Cooling the nozzle side wall or edge, however, is quite difficult in view of the undercut region which spaces the side wall or edge a substantial distance from the nearest apertures of the impingement plate. This substantial distance from the impingement cooling flow reduces the effectiveness of the cooling of the nozzle side wall. It is therefore very important to minimize the impingement distance, i.e., the distance between the apertures through the impingement plate and the surface to be cooled. It is also necessary to minimize that distance in a producible production nozzle segment.
In accordance with the present invention, improved side wall fabrication and cooling is provided. Particularly, with the weld joint between the cover and the nozzle side wall located remotely from the hot gas path through the turbine, side wall cooling is improved by providing the inturned flange of the nozzle side wall with a plurality of openings in which apertured cooling plates are received. The apertures through the plates extend between opposite edges of the plate. Consequently, by inserting the plate into openings along the inturned flange of the side wall, one edge of the plate lies in communication with the first cavity, while the opposite edge lies in communication with the second cavity. Most importantly, however, the ends of the cooling flow exit apertures lie very close to the side wall. Consequently, cooling medium flow from the first cavity through the apertures into the second cavity exits the apertures at locations directly adjacent the side wall of the nozzle to be cooled. The free flow of the cooling medium therefore travels a limited distance insufficient for the flow exiting the apertures to spread. Also, because the apertures are elongated, i.e., extend between opposite edges of the plate, the cooling medium flowing from the apertures onto the side wall is more directed and focused, thereby increasing cooling efficiency.
In a preferred embodiment according to the present invention, there is provided for use in a gas turbine, a nozzle segment having outer and inner band portions and at least one vane extending between the band portions, at least one of the band portions including a nozzle wall defining in part a hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therebetween and an impingement plate disposed in the chamber defining with the cover a first cavity on one side thereof for receiving a cooling medium, the impingement plate on an opposite side thereof defining with the nozzle wall a second cavity, the impingement plate having a plurality of apertures therethrough for flowing cooling medium from the first cavity into the second cavity for impingement cooling the nozzle wall, the nozzle segment including a side wall extending generally radially between the nozzle wall and the cover, means carried by the segment having a plurality of apertures therethrough for flowing the cooling medium from the first cavity for impingement cooling the side wall of the nozzle segment.
In a further preferred embodiment according to the present invention, there is provided for use in a gas turbine, a nozzle segment having outer and inner band portions and at least one vane extending between the band portions, at least one of the band portions including a nozzle wall defining in part a hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therebetween and an impingement plate disposed in the chamber defining with the cover a first cavity on one side thereof for receiving a cooling medium, the impingement plate on an opposite side thereof defining with the nozzle wall a second cavity, the impingement plate having a plurality of apertures therethrough for flowing cooling medium from the first cavity into the second cavity for impingement cooling the nozzle wall, the nozzle segment including a side wall extending generally radially between the nozzle wall and the cover and having an inturned flange defining an undercut region adjacent the side wall, the impingement plate having an edge secured to the inturned flange, the inturned flange having at least one slot therethrough between the first cavity and the undercut region, a plate disposed in the slot and extending into the undercut region, and a plurality of apertures passing through the plate for flowing the cooling medium from the first cavity into the undercut region for impingement cooling the side wall of the nozzle segment.